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Forces and Motion

When does Momentum Change?

The law of conservation of momentum only applies if there are
no external forces. An example of an external force is friction.
Friction is a resultant force that makes
objects slow down and stop if there is no force to balance it.

Newton's second law tells us that a resultant force will cause
the motion (and therefore the momentum) of an object to change.

What is the Equation for a Change in
Momentum?

Force = change in momentum ÷ time taken for the change.

This equation is written as
F = (mv - mu) ÷ t

where F = Force
mv = final momentum (the one it ended up with)
mu = initial momentum (the one it started with)
t = time

This equation can be rearranged to give

Change in momentum = Force x time.
or
mv - mu = F x t

The units of momentum can be therefore be written as
Ns called Newton seconds (Force x time) as well as kgm/s.

An example of a change in momentum is a moving car
being stopped by using its brakes or by crashing into a wall.

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